To be able to connect various application programs of personal computers (PC) with a PC card for an integrated services digital network (ISDN), an application program interface (API) is needed. Such an application program interface is to make it possible to use PC application programs independently of the type and manufacturer of the PC card for the ISDN network ("ISDN-Wann geht die Puste aus?", Funkschau, No 23, 1989, pages 24-31).
A prior art application program interface, the so-called Common ISDN API, ensures that PC application programs (e.g., for file transfer, terminal emulation) are adapted to the ISDN environment independently of the PC ISDN card. This application program interface, however, is not yet usable internationally for different ISDN protocols for the ISDN D channel ("PC-Kommunikation, Ende des Chaos?", Funkschau, No. 8, 1989, pages 26-30).
The Common ISDN API is designed specifically for the ISDN protocol 1TR6 of the German Bundespost, so that for protocols in other countries, such as France or the USA, the application programs have to be adapted.